Nuclear reactors such as pressurized water nuclear reactors include a vessel which contains the core of the nuclear reactor which consists of fuel assemblies and through which, in service, the cooling fluid of the reactor flows.
The core of the reactor consists of fuel assemblies which are generally of a right prismatic shape, which rest on a core support plate via their lower part or bottom nozzle and which are placed in a vertical arrangement. The fuel assemblies are juxtaposed and constitute a dense arrangement in which each of the fuel assemblies is in contact with adjacent assemblies, in a lattice arrangement, via its nozzles and its spacer grids.
The fissile fuel material contained in the fuel assemblies is progressively consumed in the nuclear reactor in service, so that the fuel assemblies are progressively depleted in fissile fuel material and consequently undergo a form of wear.
It is therefore necessary periodically to carry out refuelling operations of the core of the nuclear reactor. These operations, which require shutdown and cooling of the nuclear reactor, are generally carried out on a fraction of the core of the reactor, so as to optimize the use of fuel.
Furthermore, when first commissioning a nuclear reactor, it is necessary to fuel the core with new fuel assemblies which constitute the first charge of the nuclear reactor.
The operations of fuelling or refuelling a nuclear reactor are carried out under water, with the vessel head being dismounted, from the upper level of the cavity of the reactor, in the bottom of which the reactor pit opens out.
In order to carry out fuelling and refuelling operations, use is made of a fuel assembly lifting and handling machine, called a fuelling machine, which includes horizontal guide means arranged above the upper level of the cavity and a carriage mounted movably on the guide means, in at least two directions of a horizontal plane, so as to be capable of placing fuel assembly gripping and lifting means in line with each of the fuel assembly positions in the core of the nuclear reactor.
The fuel assembly gripping and lifting means include a tubular external shaft of generally cylindrical shape, fastened on the carriage with its axis vertical, and a cylindrical internal mast in a coaxial arrangement and mounted movably in the axial direction, inside the external shaft.
In order to fit the fuel assemblies in the core of the reactor where these assemblies are placed contiguously in a lattice, it is necessary to provide extremely precise handling and lifting means.
In particular, the internal mast of the fuelling machine must make it possible to displace the fuel assemblies along a perfectly defined vertical axial direction. For this purpose, the mobile internal mast of the fuelling machine generally includes longitudinal slideways which interact, during displacement of the internal mast, with sets of rollers having axes perpendicular to the axial direction of the external shaft and of the internal mast and which are arranged in a plurality of groups aligned in directions parallel to the axis of the external shaft and of the mobile internal mast.
The displacements in the axial direction, i.e. in the vertical direction, of the mobile mast for carrying out the core refuelling operations are of very high amplitude and must be carried out with very high precision with respect to the alignment of the direction of displacement of the mobile mast with the axes of the assembly positions in the core of the reactor being fuelled.
The guide groups for the mobile mast which are carried by the external shaft are very long and must therefore have perfectly straight alignment axes which are perfectly defined in orientation and in position.
Before start-up of the fuelling machine, or even during use, it may be necessary to check and adjust the alignment of the mobile mast guide means carried by the external shaft.
These adjustments may be required, in particular, by loss of adjustment of the roller groups of the guide devices or by deformation of the external shaft, for example due to an impact.
This operation, which is carried out in the reactor building, in proximity to the reactor pit, requires scaffolding to be installed in order to be able to access the various roller positions along the external shaft.
Adjustment is carried out by means of eccentrics which are arranged at the level of each roller support which is accessible through an opening provided in the external shaft, at the level of the roller support.
Such an operation, which includes mounting and dismounting scaffolding, may require a length of the order of 48 hours, during which the fuelling machine is unavailable, which commensurately lengthens the total time of the shutdown of the reactor for refuelling.
In addition, the operations of adjusting the guide elements of the mobile mast of the fuelling machine inside the cavity of the reactor require this part of the cavity to be emptied, after fitting a gate.